disk: sdmmc: support L4 series with shared DMA channel

Update the driver to support DMA operations on L4 series devices, with
a shared DMA channel. Split channels do not work on these chips, since
there is no dedicated TX and RX channels on the DMA, so configuring two
channels with SDMMC as the peripheral results in a non-functional
configuration.

Fixes #91216.

Signed-off-by: Jordan Yates <jordan@embeint.com>